1. Field of Invention
The present invention relates generally to radiant energy responsive apparatus and in particular to infrared radiation homing apparatus and a radiation sensing device useful therein.
Radiation homing or tracking apparatus is useful to track or home in on an optical image of a remote light transmitting object. In one application the object is a missile or aircraft and the transmitted light takes the form of an infrared radiation (hereinafter referred to as IR) pattern emitted from the propulsion system thereof. Since many natural objects and various kinds of machines also emit IR, the tracking apparatus must be capable of discriminating between the radiation pattern of the object being tracked and the IR pattern of the background as well as the IR patterns of undesired or false targets, such as pyrotechnic devices or flares. In addition, the radiation detection apparatus should be capable of operation in a multiple target environment so that it tracks or homes in on a single true target.
2. Prior Art
It is generally known that the IR patterns of a true target, such as a missile, or aircraft and of a false target, such as a flare, have different spectral energy content in different wavelength bands of the spectrum. That is, a flare and an aircraft have different IR color temperatures. Low color temperature targets, such as missiles or aircraft, have their maximum spectral energy emitted at a longer wavelength than higher color temperature targets, such as flare decoys. As a result, many devices are known which discriminate between true and false targets by comparing the differences between the IR intensities in different wavelength bands with one another or with reference intensities.
One type of prior art device includes a separate electro-optical system for each particular color. The IR color intensities produced by each system are balanced with one another to reduce the effect of false target radiation (U.S. Pat. No. 2,927,212) or simply employed to enable or disable (U.S. Pat. Nos. 3,147,381 and 3,219,826) the electro-optical system associated with the true target IR color. The use of a separate optical system for each color is undesirable because the parallel optical systems are bulky and heavy. In addition, these parallel electro-optical systems are not only difficult to align so that the object viewed by each is correspondingly positioned but also difficult to design so that they track with variations in ambient light level and ambient temperature.
In another prior art device, exemplified by U.S. Pat. No. 3,160,751, a single optical system is employed to discriminate between different IR patterns. In this device a number of concentric color filters are employed at the optical entrance aperture to permit preselected and separate IR colors to enter the optical system. A field lens images the entrance aperture and, hence, the filtered IR colors, on a like number of detectors such that separate detectors receive different IR colors. The output voltages of the detectors are then compared to produce an indication or identification of the target from which the IR is emanating. A disadvantage of this device is that it requires a plurality of detectors, one for each IR color being measured. Moreover, this device does not appear to give a valid color identification in a multiple target environment or select and track a single target in such an environment.